Post-growth Annealing of Bridgman-grown CdZnTe and CdMnTe

16 Pages·2015·0.29 MB·English

BNL-108270-2015-JA
Post-growth Annealing of Bridgman-grown CdZnTe
and CdMnTe Crystals for Room-temperature
Nuclear Radiation Detectors
Stephen U. Egarievwe
Submitted to SORMA XV Conference Proceedings
August 2015
Nonproliferation and National Security Department
Brookhaven National Laboratory
P.O. Box 5000
Upton, New York 11973
bnl.gov
U.S. Department of Energy
Office of Nonproliferation and Verification Research and
Development (NA-22)
Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under
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Post-growth Annealing of Bridgman-grown CdZnTe and CdMnTe Crystals
for Room-temperature Nuclear Radiation Detectors
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Stephen U. Egarievwe a,*, Ge Yang b, Alexander A. Egarievwe a, Ifechukwude O. Okwechime a,
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Justin Gray a, Zaveon M. Hales a, Anwar Hossain b, Giuseppe S. Camarda b, Aleksey E.
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Bolotnikov b, and Ralph B. James b
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a Nuclear Engineering and Radiological Science Center, Alabama A&M University, Normal 35762, USA
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b Nonproliferation & National Security Department, Brookhaven National Laboratory, Upton, NY 11973,
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USA
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Abstract
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Bridgman-grown cadmium zinc telluride (CdZnTe or CZT) and cadmium manganese telluride (CdMnTe
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or CMT) crystals often have Te inclusions that limit their performances as X-ray- and gamma-ray-
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detectors. We present here the results of post-growth thermal annealing aimed at reducing and
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eliminating Te inclusions in them. In a 2D analysis, we observed that the sizes of the Te inclusions
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declined to 92% during a 60-hour annealing of CZT at 510 oC under Cd vapor. Further, tellurium
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inclusions were eliminated completely in CMT samples annealed at 570 oC in Cd vapor for 26 hours,
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whilst their electrical resistivity fell by an order of 102. During the temperature-gradient annealing of
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CMT at 730 oC and an 18 oC/cm temperature gradient for 18 hours in a vacuum of 10-5 mbar, we observed
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the diffusion of Te from the sample, so causing a reduction in size of the Te inclusions. For CZT samples
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annealed at 700 oC in a 10 oC/cm temperature gradient, we observed the migration of Te inclusions from a
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low-temperature region to a high one at 0.022 µm/s. During the temperature-gradient annealing of CZT in
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a vacuum of 10-5 mbar at 570 oC and 30 oC/cm for 18 hours, some Te inclusions moved toward the high-
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temperature side of the wafer, while other inclusions of the same size, i.e., 10 µm in diameter, remained
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in the same position. These results show that the migration, diffusion, and reaction of Te with Cd in the
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matrix of CZT- and CMT- wafers are complex phenomena that depend on the conditions in local regions,
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such as composition and structure, as well as on the annealing conditions.
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Keywords: CdZnTe and CdMnTe, Nuclear radiation detectors, Te inclusions, Thermal annealing in Cd
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vapor, Te diffusion and migration, Temperature gradient annealing
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1. Introduction

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Cadmium zinc telluride (CdZnTe or CZT) and cadmium manganese telluride (CdMnTe or CMT) are
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some of the semiconductor materials that have proven to be good candidates for applications as room-
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temperature nuclear-radiation detectors [1-4]. CZT has found usages in medical imaging [5] and in
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astrophysics for measuring celestial gamma-ray photons [6]. Bridgman-grown CZT and CMT crystals
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often contain Te inclusions that limit their performances as X-ray- and gamma-ray-detectors [7]. These
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Te inclusions form from the evaporation of Cd (due to the high pressure), and the deposition of
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segregated Te from the decomposition of the melt. Te inclusions were observed to act as localized
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centers where different impurities gather, and may affect electrical compensation and cause charge
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trapping [8]. The trapping of charge carriers generated by ionizing radiation entails fluctuations of the
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collected charges, so degrading the energy resolution of thick (> 5-15 mm) detectors [9, 10]. While
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efforts have been made to control the formation of Te inclusions in Bridgman-grown CZT and CMT, their
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presence persists because the crystals are grown in Te-rich environments to ensure the high resistivity
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needed for the detector’s room-temperature operation. Post-growth thermal annealing is a reasonable
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alternative to removing these performance-limiting Te inclusions.
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There are two basic approaches to the post-growth thermal annealing of CZT- and CMT-crystals.
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The first one involves annealing the material at elevated temperatures in a Cd-rich atmosphere. Then, Cd
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diffuses within the sample and reacts with the excess Te, so replacing the Te-rich particles with
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crystalline material. The strain introduced by forming new CZT in place of Te particles may increase the
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number of dislocations in the material. This procedure also reduces the material’s resistivity. Reportedly,
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annealing in a combination of Cd-rich and Zn-rich atmospheres maintains high resistivity [11]. The
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second thermal-annealing approach involves inducing movement of the Te inclusions under temperature
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gradients and their accumulation at the sample’s high-temperature side, leaving the higher-quality regions
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of the crystal with fewer smaller particles.
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In previous experiments we recorded that annealing in a Cd vapor effectively removes Te inclusions
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from CZT, and there was migration of Te inclusions for annealing in a temperature-gradient field [12].
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While many of the small-size Te inclusions were eliminated after annealing in a Cd vapor at 923 K for 24
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hours, the lager inclusions remained; and the size-reduction for larger inclusions become possible in two
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cases: 1) annealing at higher temperature of 1100 K for a shorter time of 60 minutes, and, 2) a two-cycle
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annealing at 1100 K for 15 minutes per cycle [12]. It has been reported that annealing under a Cd vapor
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pressure at high temperatures above ~1170 K generates a large quantity of irregular Cd inclusions in the
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CZT matrix [13]. Thus, depending on the crystal composition and structure, annealing at lower
temperatures and longer time could have the advantage of less heat stress on the CZT matrix
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compared to annealing at high temperatures. In the present work, we show that both elimination of
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small-size Te inclusions and the size-reduction of large Te inclusions in CZT wafers can also be
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achieved at lower temperature and longer time of up to 60 hours. We also present further results of
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post-growth annealing of CZT- and CMT- crystals under Cd vapor and a temperature gradient. The
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discussion of the new results focused on 1) the Te diffusion in the detector material, 2) the reduction in
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the size of Te inclusions, 3) the migration of Te inclusions, and, 4) the resistivity of the wafers.
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2. Experimental Procedures
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Wafers were cut from Bridgman-grown CZT- and CMT-ingots ranging from 2.5 mm- to 5.25 mm-
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thick, 5.0 mm to 10.4 mm long, and 5.0 mm- to 7.6 mm-wide. The samples were mechanically polished
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with 800-grit and 1200-grit silicon carbide abrasive papers, and then polished on multi-tex pads with
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alumina powder (Al O ) of decreasing size from 3- to 0.1-microns. Each sample was sealed in a quartz
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ampoule under vacuum of 10-5 mbar. For annealing under Cd vapor, we placed Cd at one end of the
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ampoule and sealed it with the sample. The sealed ampoules were placed in a predetermined region of an
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annealing furnace. In each annealing run, the location of the ampoule was determined by setting the
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heating elements to give a furnace-temperature profile that would give the desired annealing temperature,
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the Cd temperature for annealing under a Cd atmosphere, and a temperature gradient for temperature-
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gradient annealing. Fig. 1 shows an example of the profile of the furnace’s temperature used in this
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experiment. For annealing under Cd vapor, the distance between the sample and the Cd material was
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determined from the profile of the furnace’s temperature and the desired temperatures for the sample and
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Cd.
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3
780
770
C) 760
o
(
e 750
r
u
at740
r
e
p730
m
e720
T
710
700
690
0 1 2 3 4 5 6 7 8 9 10 11 12
Position (cm)

Fig. 1. An example of a profile of the furnace temperature used in this experiment. The top of the profile
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could be moved up or down to get the desired annealing-temperature and temperature-gradient.
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To study diffusion of Te inclusions, the reduction in their size, and their migrations, we used infrared
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(IR) transmission microscopy to image them both before and after annealing; also, we then obtained the
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resistivity of the samples by current-voltage (I-V) measurements. The matrix of the CZT- and CMT-
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crystals is transparent to IR light, while the Te inclusions are opaque. The samples were marked to track
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the locations and orientations of the Te inclusions that we studied. For I-V measurements, planar Au
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electrical contacts were placed on two opposite surfaces of the rectangular-shaped samples by an
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electroless chemical-deposition method, wherein AuCl 5% solution droplets are pipetted on to the
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samples’ surface and the excess solution removed via a liquid-absorbing soft felt paper. The I-V
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measurement system consists of a sample mount enclosed in an aluminum box fitted with electrical leads
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connected to a Keithley 237 high-voltage source and a low-current measure unit.
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After I-V measurements and before sealing the samples in ampoules for annealing, the Au electrical
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contacts were removed by polishing with a 1200-grit silicon-carbide abrasive paper, followed by
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polishing on multi-tex pads with Al O powder of decreasing size from 3- to 0.1- microns. Polishing was
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repeated after annealing to remove any rough surfaces caused by heating the samples. Smoothing the
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surface allows a more uniform entry of IR light into the matrix of the CZT and CMT-samples while
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preventing the image of a rough surface from obscuring images of Te inclusions. The IR images were
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taken with a Nikon industrial microscope Eclipse LV100 fitted with an IR light source, an IR camera, a
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motorized xyz-translation stage, and image-capturing and analysis software. The following sets of
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annealing experiments were carried out:
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1. Annealing of CZT at 510 oC under Cd vapor for 60 hours, after which the reduction in size of Te
inclusions was measured.
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2. Annealing of CMT crystals at 570 oC in Cd vapor for 26 hours. The elimination of Te inclusions
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and change in resistivity were studied.
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3. Temperature-gradient annealing of CZT at 30 oC/cm and CMT at 18 oC/cm under a 10-5 mbar
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vacuum for 18 hours at 570 oC and 730 oC , respectively. This set of experiments yielded details
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of the diffusion and migration of Te inclusions.
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4. Annealing of CZT at 700 oC under Cd vapor at 650 oC for 30 minutes at 10 oC/cm temperature
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gradient. The migration of the Te inclusions under temperature gradient was measured.
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3. Results and Discussion
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The results are discussed under three categories: Thermal annealing in Cd vapor; annealing under a
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temperature gradient; and, temperature-gradient annealing in Cd vapor.
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3.1. Thermal Annealing in Cd Vapor
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In previous experiments we recorded that many of the small-size Te inclusions were eliminated after
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annealing in a Cd vapor at 923 K (~ 650 oC) for 24 hours while the lager inclusions remained, and
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observed size-reduction for larger inclusions in two cases: 1) annealing at higher temperature of 1100 K
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(~ 827 oC) for a shorter time of 60 minutes, and, 2) a two-cycle annealing at 1100 K for 15 minutes per
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cycle [12]. The results in the present work show that both elimination of small-size Te inclusions and
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the size-reduction of large Te inclusions in CZT wafers can also be achieved at lower
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temperature of 510 oC and longer time of 60 hours – see Table 1 and Fig. 2 which showed up to
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96% size reduction of large inclusions in a 2D analysis. Annealing at this relatively low
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temperature and longer time reduces the generation of large quantity of irregular Cd inclusions
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reported for annealing at much higher temperatures above ~1170 K [13].
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Table 1 shows the size reductions for seven selected Te inclusions in CZT annealed at 510 oC for 60
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hours in Cd vapor. The areas of the Te inclusions were calculated using NIS-Elements Nikon imaging
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software. This reduction could result from Te diffusing outside the CZT matrix, or the Te reacting with
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Cd that diffuses within the CZT matrix from the Cd-rich atmosphere, thus replacing the excess Te with
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crystalline material. The dominant cause of size reduction in this case is the reaction of Cd with Te as
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illustrated in Fig. 2 that shows the almost linear dependence of the size-reduction on the pre-annealing
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sizes of the Te inclusions, i.e., larger inclusions provide a larger surface area for reaction with Cd. The
percent size reduction is almost constant, averaging 92%.
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Table 1
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Percentage size reduction of Te inclusions in CZT annealed at 510 oC for 60 hours in Cd vapor.
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Before annealing (µm2) After annealing (µm2) Size reduction (%)
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311.2 24.6 92.1
235.2 9.1 96.1
359.8 37.4 89.6
280.7 21.2 92.4
196.1 29.2 85.1
311.2 16.6 94.7
333.4 15.9 95.2

154 400
Reduction in size
350
%) Percent size reduction
r 300
o
2
m
250
µ
(
(
n 200
o
ti
uc150
d
e
r
e 100
z
Si
50
0
180 220 260 300 340 380
Te inclusion size (µm2)

Fig. 2. Linear dependence of size-reduction on the pre-annealing sizes of the Te inclusions – larger Te
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inclusions provide larger surface area for reacting with Cd. The percent size reduction is approximately
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constant.
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Depending on the annealing temperature and the Cd-rich atmosphere, increasing the annealing time
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could completely eliminate the Te inclusions. Figure 3 shows the IR images of CMT before and after
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annealing in 570 oC in Cd vapor for 26 hours, at which the Te inclusions were completely eliminated
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through the reaction of Cd and Te. The electrical resistivity of the CMT sample was reduced by an order
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of 102, as shown by the I-V curves in Fig. 4. Similar reductions in the electrical resistivity of CZT
annealed in Cd vapor were reported in the literature, where a second step of annealing in Te-rich
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atmosphere was used to restore the resistivity [12]. During annealing in Cd-rich atmosphere, extra Cd
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interstitials are introduced into the CZT matrix, a process which may cause a loss of compensation
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between deep-level Te antisites, Cd interstitials, and the Cd vacancies that are responsible for the high
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resistivity in CdTe and its associated tertiary compounds, CdZnTe and CdMnTe [12,13]. The result is a
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shift of the Femi level towards the bottom of the conduction band or to the top of the valence band [12].
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Much success was reported in increasing the resistivity of CZT by annealing in Te-rich atmosphere [14];
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in some cases with restoration to 109 Ω-cm from the range of 103 and 105 Ω-cm [15].
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Post-growth annealing processes lowered the resistivity of the CZT- and CMT-materials. This effect,
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illustrated in Fig. 4, compares the current-voltage measurements of Bridgman-grown and modified
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floating-zone-grown CMT crystals before and after annealing. The results of current-voltage
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measurements on CMT wafer annealed at 570 oC in Cd vapor for 26 hours showed a reduction in
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electrical resistivity by an order of 102.
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(a) Before annealing (b) After annealing

Fig. 3. Te inclusions are completely eliminated from a CMT wafer annealed at 570 oC in Cd vapor for 26
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hours.
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7
600
)
A
(n 400
t
n
e
r 200
r
u
C
0
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
-200
Voltage (V)
-400
-600 Before annealing
After annealing
-800
-1000
-1200
-1400

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Fig. 4. I-V curve of CMT wafer annealed at 570 oC in Cd vapor for 26 hours, showing a reduction in
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electrical resistivity by an order of 102.
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